The concept of wireless power has been developed for a long time. It is only in recent years, however, with the rapid development of the microprocessor, that wireless power has become a viable solution. Today, wireless technology is growing at an exponential rate, with everything from phones to consumer electronics being wirelessly connected. Despite the rapid development in the technology, battery life of these devices remains a problem. Wireless power or wireless charging is designed to solve these problems.
Wireless power is also known as inductive charging. It requires two coils, a transmitter coil and a receiver coil. An alternating current passes through the transmitter coil, generating a magnetic field. The magnetic field induces a voltage in the receiver coil, which can be used to power an external load, such as to power a mobile device or to charge a battery.
In a wireless power/wireless charging system, a transmitter is connected with a power source. The transmitter contains a primary coil that generates a magnetic field. When a receiver, which has a secondary coil, makes contact or is in close proximity to the transmitter, the transmitter and the receiver are magnetically coupled. Power transfers from the transmitter through coupled inductors, such as an air core transformer. The receiver takes the inputs from the secondary coil, and passes it through a rectifier circuit. For efficiency purposes, the rectifier circuit is normally a controlled full bridge rectifier, which uses field-effect transistors (FETs). A bootstrap circuit powers the high side FETs in the rectifier circuit. The bootstrap circuit powers the high side FETs using two external capacitors, which are connected to the integrated circuit through two high side bootstrap terminals.
In modern integrated circuit designed for wireless power devices, the amount of power transferred is controlled by internal control circuits. Control signals are transmitted from the receiver to the transmitter based on detected conditions at the receiver to increase or decrease power. Further, the receiver monitors receiver conditions and triggers internal protection mechanisms.
One of the internal protection mechanisms in the receiver circuit is an over voltage protection (OVP) circuit. The receiver monitors the output voltage. When the output voltage reaches a level that is greater than a predefined value, an error signal is generated. Upon detecting of the OVP condition, an OVP circuit is enabled to clamp the input voltage so that the output voltage can drop to the predefined level. Upon the output voltage returning to the predefined level, the OVP circuit is disabled and the receiver returns to regular operation mode. Using two capacitors to clamp the AC input is a proven solution for wireless power circuit. The dual capacitor OVP circuit uses capacitor OVP circuit uses direct capacitive coupling across the inputs to clamp the input. Typically, two external capacitors are needed for the OVP circuit to clamp the inputs terminals, and two OVP terminals are used for the OVP circuit to connect to two external OVP capacitors.
In addition to over voltage protection, a wireless power integrated circuit may also include a separate output shutdown switch to shut off the output when certain predefined conditions are detected. The output shutdown switch, also known as a load switch, can be a P-type switch, such as a PMOS, which is easy to drive. However, a PMOS is about twice the price of an N-type switch, such as an NMOS for the same Drain-Source On-Resistance (RDS(on)) and Voltage rating. It is very beneficial to change this to an NMOS. However, to get the lowest RDS (on) possible, the gate of the NMOS needs to be driven some voltage above the supply to fully enhance the NMOS.
There are two traditional designs to drive the gate of the NMOS so that the less expensive NMOS can be used as the output shutdown switch. The first is to use an internal charge pump that drives the NMOS shutdown switch. The internal charge pump, however, requires a large switching capacitor. It also requires a large die area. The second solution is to use an external charge pump that does not require a large die area. However, an external charge pump requires two terminals with High Voltage Electrostatic Discharge (ESD) cells, additional external components and more pins on the package. Improvements are needed for the integrated circuit for a wireless power receiver.
FIG. 1 shows prior art circuit diagram of an integrated circuit 1 for a wireless power receiver.
Integrated circuit 1 has two input terminals RX1 11, RX2 12, and a ground terminal GND 10. Series capacitor 3 and parallel capacitor 4 make up the dual resonant circuit with a secondary coil 2. Secondary coil 2 receives power from a power-transmitter coil in a power transmitter unit and passes through the secondary dual resonant circuit. The Dual resonant circuit enhances the power transfer efficiency and enables a resonant detection method.
A full bridge rectifier circuit 40, coupled between an input terminal RX1 11 and an input terminal RX2 12, provides full-wave rectification of the AC waveform received from RX1 11 and RX2 12. The output of rectifier circuit 40 is connected to a rectifier output terminal RECTO 15 and terminal GND 10.
A bootstrap circuit 30 is used to power rectifier circuit 40. Two external bootstrap capacitors, bootstrap capacitor 5 and bootstrap capacitor 6 are connected to a bootstrap terminal HSB1 13 and a bootstrap terminal HSB2 14. A low voltage power, e.g. 5-volt, charges the bootstrap capacitors through a bootstrap diode 31 and a bootstrap diode 32, respectively. The bootstrap circuit, therefore, provides power to high side switches of rectifier circuit 40 in normal operation.
Integrated circuit 1 in FIG. 1 also has an OVP circuit 20 that includes an OVP switch 21 and an OVP switch 22. OVP switch 21 is coupled between an OVP clamping terminal CLMP1 25 and the ground. OVP switch 22 is coupled between an OVP clamping terminal CLMP2 26 and the ground. A signal OVP 29 drives OVP switch 21 and OVP switch 22. An external OVP capacitor 23 is coupled between RX1 11 and CLMP1 25. Another external OVP capacitor 24 is coupled between RX2 12 and CLMP2 26. When signal OVP 29 is asserted asserted upon detecting an OVP condition, OVP switch 21 and OVP switch 22 turn on and pull CLMP1 25 and CLMP2 26 to the ground. External OVP capacitors 23 and 24 provide capacitive coupling between RX1 11 and RX2 12 and therefore clamp the input. The output voltage at RECTO 15 will drop back to the predefined level accordingly.
The prior art provides over voltage protection. However, the OVP circuit requires two additional capacitors and two additional terminals. Although integrated circuit 1 provides a working solution for over voltage protection, it requires additional capacitors and terminals.
FIG. 1 also shows an output shutdown circuit 50. Output shutdown circuit 50 detects different signals and outputs an output shutdown signal when one or more predefined conditions are met. An external load switch 7 is coupled between a load switch (LSW) terminal 16 and RECTO 15. Load switch 7 can be either an N-type switch or a P-type switch. An N-type switch includes, but is not limited to, NMOS or NPN. A P-type switch includes, but is not limited to, PMOS. A P-type switch, which uses a simpler gate drive, is much more expensive than an N-type switch. An N-type switch, however, requires gate drive that drives the voltage above the supply. As shown in FIG. 1, a charge pump 51 is used to provide gate drive to N-type load switch 7. A charge pump capacitor 52 is needed to power load switch 7. This design requires an additional capacitor 52 and takes a lot of die area. Capacitor 52 can be externally connected to integrated circuit 1. To be able to connect capacitor 52 externally, two additional terminals are needed. Either an internal charge pump or an external charge pump requires additional capacitors. It also requires either a larger die area or two additional terminals.
Methods and structures for improving such wireless power receiver are sought.